1. Field of the Invention
This invention pertains generally to current-limiting devices and, more particularly, to current-limiting devices including a current-limiting material, such as a conductive polymer, which exhibits a sharp increase in electrical resistance at a threshold current, and also including electrodes, which electrically engage the current-limiting material.
2. Background Information
Current-limiting polymer compositions, which exhibit positive temperature coefficient (PTC) resistive behavior, and electrical devices comprising current-limiting polymer compositions have been widely used. See, for example, U.S. Pat. No. 5,614,881. The current-limiting polymer compositions generally include conductive particles, such as carbon black, graphite or metal particles, dispersed in a polymer matrix, such as a thermoplastic polymer, elastomeric polymer or thermosetting polymer. PTC behavior in a current-limiting polymer composition is characterized by the material undergoing a sharp increase in resistivity as its temperature rises above a particular value known as the switching temperature. Materials exhibiting PTC behavior are useful in a number of applications such as, for example, electrical circuit protection devices, in which the current passing through a circuit is controlled by the temperature of a PTC element forming part of that circuit.
Electrical circuit protection devices comprising current-limiting polymer compositions typically include a current-limiting polymer device having two electrodes embedded in a current-limiting polymer composition. When connected to a circuit, the circuit protection devices have a relatively low resistance under normal operating conditions of the circuit, but are tripped, that is, converted into a high resistance state, when a fault condition or persistent overcurrent condition occurs. Under such conditions, when the circuit protection device is tripped, the current passing through the PTC element causes it to resistively self-heat to its switching temperature, Ts, at which a rapid increase in its resistance takes place.
The residual current, which flows through the current-limiting device, allows a series circuit breaker to absorb any stored residual energy (e.g., the majority of such energy is absorbed by the circuit breaker arc chamber during the switching transient and during recovery/reclosing to reestablish the power distribution system voltage) in the power distribution system. Typically, an external current-limiting device engages the load-side terminals of the circuit breaker. For example, a conductive polymer of the current-limiting device is coupled in series with the mechanical circuit breaker separable contacts, in order to limit fault current as those contacts open.
Previous materials used for current-limiting applications in conjunction with low voltage circuit breakers (e.g., less than about 600 VAC) generally consisted of a very brittle blend of conductive filler (i.e., carbon black) of a thermoplastic binder with two spring-loaded metal plates employed as electrodes. These electrodes serve to allow current to flow through the current-limiting material. In this arrangement, approximately 80% of the total device resistance resulted from contact resistance, while only about 20% resulted from bulk material resistance.
U.S. Pat. No. 4,800,253 discloses a metal electrode, such as an electrodeposited copper or nickel foil electrode having a microrough surface, which is in direct physical contact or which is secured to the surface of a conductive polymer element. U.S. Pat. No. 4,800,253 states that columnar electrodes, such as solid or stranded wires, having a microrough surface are embedded in a PTC conductive polymer.
Known high power prior art current-limiting devices, for example, up to about 600 VAC with a rated current of greater than several amperes (e.g., about 10 A to about 63 A), which employ current-limiting polymers (see, e.g., U.S. Pat. No. 5,861,795), also employ a parallel electrical (e.g., wire) shunt to protect the current-limiting material from overvoltage and from the stored system energy (e.g., generally magnetic system energy resulting from system inductance). However, the shunt increases the volume of the package for the current-limiting device and the cost of the overall device.
Typically, relatively low power prior art current-limiting devices, for example, up to typically about 12-24 VDC or higher, with a rated current of less than several tens of milliamperes, which employ current-limiting polymers, do not employ a parallel electrical shunt to protect the current-limiting material.
There is room for improvement in terms of the current-limiting material and electrodes employed in current-limiting devices.
The present invention provides improvements in the operation of current-limiting devices by providing a non-uniform pressure distribution between one or both of the first and second electrodes and the current-limiting material therebetween. in accordance with the invention, a current-limiting device comprises a current-limiting material; first and second electrodes structured for carrying current through the current-limiting material, with the first electrode electrically engaging a first portion of the current-limiting material, and the second electrode electrically engaging a second portion of the current-limiting material; and means for providing a non-uniform pressure distribution between at least one of the first and second electrodes and the current-limiting material.
The means for providing a non-uniform pressure distribution preferably includes a spring having a predetermined spring rate of about 100 to about 7000 pounds per inch. Highly preferred predetermined spring rates range from about 100 to about 700 pounds per inch, with a predetermined spring rate of about 300 pounds per inch being especially preferred.
Preferably, the first and second electrodes are solely electrically connected by the current-limiting material.
The means for providing a non-uniform pressure distribution may include a pair of supports for edges of at least one of the electrodes, and means for applying a force to the supports, in order to provide the non-uniform pressure distribution.
As one aspect of the invention, the electrodes have a first surface and a second surface which engages the current-limiting material; and the means for providing a non-uniform pressure distribution includes a rubber spring member having a plurality of openings and positioned on the first surface of one of the electrodes, a plate positioned on the rubber spring member, and means for applying a clamping force between the plate and the first surface of the other one of the electrodes, in order to provide the non-uniform pressure distribution.
As another aspect of the invention, the electrodes may have a first portion, a second portion and a third portion. The means for providing a non-uniform pressure distribution includes a first clip and a second clip which engage the first portion and the third portion of the electrodes, respectively, in order to apply a force thereto without engaging the second portion of the electrodes, in order to provide the non-uniform pressure distribution.
Preferably, the first and third portions are side portions of the electrodes, and the second portion is an intermediate portion between the side portions.